This application is based on and incorporates herein by reference Japanese Patent Application No. 2002-122335 filed on Apr. 24, 2002.
The present invention relates to a pressure sensor in which a diaphragm is exposed to a fluid, the pressure related to which is to be measured by a sensor device. The pressure sensor is used, for example, for measuring a pressure loss by a diesel particulate filter (DPF) in an exhaust pipe of an automobile with a diesel engine.
In recent years, laws and regulations related to emission control have become more stringent, and there is a need to reduce soot from especially diesel engines. A technology that meets such a need is DPF. DPF is a system that relies on a filter for preventing soot from being released into the atmosphere. The filter is placed in an exhaust pipe to trap the soot. The trapped soot is combusted when a certain amount is accumulated. Normally, a ceramic filter is used. The volume of the trapped soot is critical for ensuring clean burning. An abnormal combustion can be caused by either too much of or too little of the soot and can lead to a problem such as a filter crack.
A method generally used today for detecting the amount of the trapped soot is based on a pressure loss by the filter. The pressure loss can be measured by either measuring only the pressure at an upstream position relative to the filter or by measuring the differential pressure between the pressure at an upstream position and that at a downstream position relative to the filter. However, when the soot is being burnt, the temperature of the gas near the DPF can rise up to 600xc2x0 C. or as high as 1000xc2x0 C. For this reason, it is not possible to directly attach a low cost pressure sensor such as a semiconductor pressure sensor, which is widely used for automotive applications, directly onto the exhaust pipe for measuring the pressure loss.
Instead, an additional pipe, such as a hose, is tapped into the exhaust pipe, and the pressure loss is measured by a pressure sensor attached to the hose at a location where the temperature of the exhaust gas is low enough.
However, because the exhaust gas, which is at a high temperature and has a high level of moisture, is cooled off, the pressure sensor is exposed to an environment with 100% humidity and condensed water. The inventors of the present invention studied the condensed water sampled from an automobile with a diesel engine. It turned out that the condensed water is very acidic having a pH as low as 2, which creates an extremely harsh environment. Pressure sensors are susceptible to corrosion in such a harsh environment with high acidity and high moistness.
The present invention has been made in view of the above aspects with an object to provide a pressure sensor suited for a use in a harsh environment for detecting the pressure of, for example, an exhaust gas.
To achieve the above object, a pressure sensor according to the present invention includes a sensor device and a diaphragm. The diaphragm is exposed to a fluid, the pressure relative to which is detected using the sensor device. The diaphragm is made of a material having a pitting index, which is defined by the equation, (Cr+3.3Mo+20N), of 50 or greater and Ni content of 30 weight % or greater to prevent the diaphragm from corroding due to the fluid.